Air fractionation plant, operating method and control facility

ABSTRACT

An air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air. The recooling apparatus is configured so as to cool the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature. A corresponding operating method and a control facility are likewise provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from European application EP 15 002 236.6 filed on Jul. 28, 2015.

BACKGROUND OF THE INVENTION

The invention relates to an air fractionation plant, a method of operating an air fractionation plant and a control facility for such an air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air, characterized in that the recooling apparatus is configured so as to cool the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature.

The production of air products in the liquid or gaseous state by low-temperature fractionation of air in air fractionation plants is known and is described, for example, in H.-W. Häring (editor), Industrial Gases Processing, Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”. The present invention is suitable for various embodiments of corresponding air fractionation plants.

Air fractionation plants have distillation column systems which can, for example, be configured as two-column systems, in particular as classical Linde double column systems, but also as three column or multicolumn systems. Apart from the distillation columns for isolating nitrogen and/or oxygen in the liquid and/or gaseous state (for example liquid oxygen, LOX, gaseous oxygen, GOX, liquid nitrogen, LIN and/or gaseous nitrogen, GAN), i.e. the distillation columns for nitrogen-oxygen separation, distillation columns for isolating further components of air, in particular the noble gases krypton, xenon and/or argon, can be provided.

The distillation column systems of air fractionation plants are operated at different operating pressures in the distillation columns thereof. Known double-column systems have, for example, a (high-)pressure column and a low-pressure column. The operating pressure of the high-pressure column is, for example, from 4.3 to 6.9 bar, in particular about 5.5 bar. The low-pressure column is operated at an operating pressure of, for example, from 1.2 to 1.7 bar, in particular about 1.4 bar. The pressures indicated here are absolute pressures in the bottom of the corresponding distillation columns. The pressures indicated will hereinafter also be referred to as “distillation pressures” because the fractional distillation of the respective fed-in air within the distillation columns occurs at these pressures. This does not rule out other pressures being able to prevail at a different place in a distillation column system.

Cooled compressed air (feed air), which is brought to pressure by means of various air compressors or combinations of various air compressors (for example main air compressors and after-compressors), is fed into the distillation column systems. All air compressors can be multistage. Since about 95% of the energy consumption in an air fractionation plant arises from the abovementioned air compressors, there is the greatest potential for savings at this place. There is fundamentally a need for more energetically efficient processes and plants for the low-temperature fractionation of air.

SUMMARY OF THE INVENTION

In the light of this background, the present invention provides an air fractionation plant, a method of operating an air fractionation plant and a control facility for such an air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air, characterized in that the recooling apparatus is configured so as to cool the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature. Embodiments of the invention are set forth in the dependent claims and the description below.

The feed air which has been brought to pressure in the air compressors of an air fractionation plant is typically recooled in differently configured cooling facilities in order to remove the heat of compression generated during compression. These cooling facilities comprise, for example, intermediate coolers and after-coolers between and downstream of one or more compression stages, as known per se. The recooling of the air compressed in a main air compressor of an air fractionation plant can, for example, be effected, inter alia, in a direct contact cooler operated using cooling water from a cooling water circuit. In addition, indirect heat exchangers which can likewise be operated using cooling water can be provided. The feed air is, in known processes, subsequently brought to very low temperatures, i.e. temperatures of significantly below 0° C., in a main heat exchanger.

The recooling is carried out, in particular, for the purpose of reducing the power consumption of the air compressors. The lower the cooling water temperature here, the further can the process air be cooled, which results in a lower power consumption of the air compressors. In addition, the process air as a result goes at a lower temperature into the actual process of air fractionation, including into the main heat exchanger. The heat to be transferred in the main heat exchanger is therefore lower, as a result of which this heat exchanger can be designed with a smaller volume and, in addition, less cold has to be generated by depressurization. Generation of cold to appropriately low temperatures of, for example, from 120 to 200 K results in considerable energy losses which are significantly greater than in the case of cooling by means of cooling water, which is carried out at close to ambient temperature. In addition, cryogenic components (heat exchangers, turbines, valves) which have to be made larger are much more costly.

Cooling water circuits of air fractionation plants typically comprise recooling apparatuses in which the warmed cooling water of the cooling water circuit is cooled by evaporative cooling by means of cooling air. In particular, cooling towers of a known type, as also explained below, can be used as recooling apparatuses. Corresponding air fractionation plants are disclosed, for example, in EP 0 644 390 A1 and JP 5 885093 A1. The cooling air used here typically originates from the surroundings of the air fractionation plant and therefore has a temperature dependent on the surroundings, a pressure dependent on the surroundings and a humidity dependent on the surroundings. The wet bulb temperature can be determined from these three parameters.

The wet bulb temperature is a measure of the cooling limit temperature, i.e. the lowest temperature which the cooling water can theoretically attain by direct evaporative cooling in a corresponding recooling apparatus. It is known that the release of water at a moist surface is in equilibrium with the water uptake capability of the surrounding atmosphere. Owing to the cold generated by evaporation, the cooling limit temperature is below the air temperature as a function of the relative atmospheric humidity. The decrease in temperature during evaporative cooling is greater, the dryer the surrounding air. The temperature difference between the wet bulb temperature and the cooled cooling water which is actually attained in a corresponding recooling apparatus is referred to in the art as cooling limit difference. The performance of a recooling apparatus, for example a cooling tower, is determined by the specific surface area of the packing, the ratio of liquid to gas and the pressure drop. In order to attain a small cooling limit difference, as is desirable in principle because of the abovementioned advantages of a lower cooling water temperature, substantially greater capital costs for erection of recooling apparatuses are therefore incurred.

The cooling limit difference used is therefore determined by economic considerations which include the aspects mentioned. In previous publications relating to forced-ventilation recooling apparatuses in industrial plants, a cooling limit difference of from 3 to 5 K is usually indicated as economically feasible, see, for example, Z. K. Morvay & D. D. Gvozdenac, “Applied Industrial Energy and Environmental Management, Part III: Toolbox—Fundamentals for Analysis and Calculation of Energy and Environmental Performance, Toolbox 12: Cooling Towers”, Chichester, Wiley, 2008. However, this figure is usually given without indicating the corresponding ambient conditions and the wet bulb temperature resulting therefrom. Recooling apparatuses having a cooling limit difference of significantly below 3 K are also technically achievable, but this is conventionally said to be uneconomical. Correspondingly lower cooling limit differences are usually used only on a laboratory scale, as is the case, for example, according to the publication by V. D. Papaefthimiou et al., “Thermodynamic Study of Wet Cooling Tower Performance”, Int. J. Energ. Res. 30(6), 2006, 411-426.

As regards further details of recooling apparatuses and the design thereof, reference may be made to relevant specialist literature, for example H.-D. Held, H.-G. Schnell, Kühlwasser: Verfahren der Systeme der Aufbereitung und Kühlung von Süβwasser, Brackwasser-und Meerwasser zur industriellen Kühlung, 5th edition, Vulkan, 2000, H. Rietschel, K. Fitzner, Raumklimatechnik, volume 2: Raumluft und Raumkühltechnik, 16th edition, Springer, 2008, J. J. McKetta, Encyclopedia of Chemical Processing and Design, volume 58, Marcel Dekker, 1997, P. N. Ananthanarayanan, Basic Refrigeration and Air Conditioning, 3rd edition, Tata McGraw-Hill, 2006, and B. Buecker, Power Plant Water Chemistry: A Practical Guide, PennWell, 1997. In particular, it may be emphasized that the cooling limit difference achievable by means of a recooling apparatus can be reliably predicted by a person skilled in the art on the basis of known calculation methods. It will therefore be said below that a recooling apparatus is configured so that it cools cooling water to a temperature which is above the wet bulb temperature by a maximum temperature value, and this is advice to a person skilled in the art to dimension a recooling apparatus so that it has the abovementioned property, i.e. has a corresponding cooling limit difference. In particular, a person skilled in the art will here take into account or provide in an appropriate way the specific surface area of packing, the ratio of liquid to gas and the pressure drop.

Surprisingly and contrary to prevailing opinion on the subject of forced-ventilation recooling apparatuses (see, for example, the above-cited publication Z. K. Morvay & D. D. Gvozdenac), it has been recognized according to the invention that, based on the overall operation costs (Total Cost of Ownership, TCO), a cooling limit difference below 3 K offers economic advantages for many air fractionation plants. Here, the cooling limit difference should be selected as a function of the capital value (in monetary units per kW, Net Present Value, NPV) at a given wet bulb temperature under design conditions. Plants having an identical capital value can thus get a recooling apparatus having the same specific quantity of heat removed independent of the respective ambient conditions. This makes systematic selection of the cooling towers as a function of the capital value possible. As mentioned, according to prevailing opinion, in particular for actual industrial applications such as air fractionation plants, a cooling limit difference significantly above 3 K is considered to be appropriate. The publication by Z. K. Morvay & D. D. Gvozdenac, which has been cited a number of times, proposes a series of efficiency improvements but not a reduction in the cooling limit difference.

The present invention therefore proposes an air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air. The air fractionation plant of the invention is characterized in that the recooling apparatus is configured for cooling the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature. In other words, a cooling limit difference of 3 K or less, in particular 2 K or less or 1 K or less, is achieved by means of the recooling apparatus of the air fractionation plant of the invention under the specified conditions.

As has been found in the context of the present invention, the statement, which is frequently to be found in the literature, that recooling apparatuses having a cooling limit difference of less than 2 K are not technically feasible is wrong. Likewise, the usual generalization that only a cooling limit difference of from 3 to 5 K is economically feasible has been found to be incorrect. An indication of the feasibility and the economic usefulness of recooling apparatuses having a fixed cooling limit difference disregarding the wet bulb temperature is, as has been recognized in the context of the present invention, not of predictive value.

A reduction in the cooling limit difference to values below 3 K enables, as has been recognized according to the invention and is documented below, the economics of air fractionation plants to be significantly improved at moderate and high wet bulb temperatures of more than 289 K. The present invention is thus based on a significant fresh evaluation of the state of knowledge of cooling tower design in respect of air fractionation processes and cryogenic processes in general.

In the context of the present invention, it has been able to be shown that an efficiently operating recooling apparatus which recools the cooling water to very close to the thermodynamically possible minimum (i.e. the wet bulb temperature) allows a significant energy saving in the air fractionation plant. This is made clear in FIGS. 2 and 3 and further details are explained. The additional capital costs (CAPEX) for an efficiently operating and thus generally larger recooling apparatus are amortized in an average of about one year by savings in the running costs (OPEX). The short amortization time of the larger recooling apparatuses is due to their small proportion (typically about 2%) of the total costs for an air fractionation plant. Table 1 gives an overview of capital costs and operating costs for a conventional recooling apparatus and a recooling apparatus according to the invention, here a corresponding cooling tower.

In an economic assessment, it is possible, for example, to look at a conventionally designed recooling apparatus in comparison with a recooling apparatus designed according to the invention as per FIGS. 2 and 3. A (forced ventilation) cooling tower having a water tank for accommodating the runback was assumed as recooling apparatus. A lower cooling limit temperature leads to a larger recooling apparatus having a likewise enlarged tank and thus to a higher capital cost. The mass flow of water is the same in both cases. The decisive fact is that in the case of a large cooling tower a larger amount of air can flow through the recooling apparatus for the same amount of cooling water; this air takes up the evaporating water and at the same time makes greater convective cooling possible. This reduces the cooling water temperature in the cooling tower according to the invention and, owing to the lower power consumption of the air compressors and an energetically optimized cooling tower, leads to lower operating costs. In the case under consideration, power costs of 0.07

/kWh were assumed in each case. The results for the recooling apparatus of an air fractionation plant using 500,000 standard cubic meters per hour of process air are reported in Table 1, with the operating costs relating to one year.

TABLE 1 Capital and operating costs of a conventional cooling tower and a cooling tower configured according to the invention According to Additional Capital costs Conventional the invention costs Tank

 50,444

 81,806

 31,362 Cooling tower

 211,500

 475,500

 264,000 Total

 295,362 According to Operating costs Conventional the invention Saving Cooling tower

 89,250

 91,630

 2,380 Water pumps

 228,985

 228,985

 0 Air compressors

 13,315,505

 13,110,446

 −205,059 Total

 −202,679

For the purposes of the present invention, use is advantageously made of a recooling apparatus which is configured in such a way that it cools the cooling water to a temperature which is at least 0.5 K, for example also at least 1 K, at least 1.5 K or at least 2 K, above the wet bulb temperature. An optimal value range for minimums and maximum cooling limit differences can be derived from the above considerations.

An air fractionation plant according to the invention can in principle have a recooling apparatus having any configuration, but this particularly preferably comprise a cooling tower. Recooling apparatuses or cooling towers having, in particular, forced ventilation are frequently used in air fractionation plants and are proven and have low maintenance requirements. A cooling tower allows, in particular, a comparatively simple reduction in the cooling limit temperature by enlargement, as explained above.

As mentioned above, the cooling water which has been cooled using the recooling apparatus is particularly suitable for after-cooling downstream of compressors in a corresponding air fractionation plant, so that the cooling water circuit of an air fractionation plant according to the invention advantageously comprises a heat exchanger which is located downstream of an air compressor or a stage of a corresponding air compressor. For the present purposes, an “air compressor” is a single-stage or multistage arrangement which is configured for increasing the pressure, in particular a radial compressor or turbocompressor. One or more heat exchangers can be present downstream of one or more compressor stages.

In the context of the present invention, a cooling zone range of the recooling apparatus can be, in particular, from 5 to 25 K, in particular from 8 to 12 K, typically about 10 K.

The invention extends further to a method of operating an air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air. The method of the invention is characterized in that the recooling apparatus is operated in such a way that, at least at a wet bulb temperature of the cooling air of more than 289 K, it cools the cooling water to a temperature which is not more than 3 K above the wet bulb temperature. The present invention likewise extends to a control facility of an air fractionation plant which is configured for carrying out a method of this type. In both cases, reference may be made to what has been said above in respect of features and advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be illustrated below with reference to the accompanying drawings which show preferred embodiments of the invention.

FIG. 1 shows an air fractionation plant according to an embodiment of the invention in the form of a schematic process flow diagram.

FIG. 2 depicts the cooling water temperatures and the corresponding wet bulb temperatures to illustrate an embodiment of the invention.

FIGS. 3A and 3B show the additional cooling of cooling water made possible according to the invention and the associated energy savings.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, an air fractionation plant according to a particularly preferred embodiment of the invention is shown in the form of a schematic process flow diagram and is designated overall by 100.

A feed air stream a is fed via a filter 101 into the air fractionation plant 100, is compressed by means of a main air compressor 102 and cooled in a direct contact cooler 103 which is supplied, inter alia, with a cooled water stream b from an evaporative cooler 104. The water stream b is introduced by means of a pump which is not separately indicated into the direct contact cooler 103. To provide the cooled water stream b, the evaporative cooler 104 is supplied with water of a stream c which can partly also be fed into the direct contact cooler 103 without prior cooling in the evaporative cooler 104. A water stream d is taken off from the direct contact cooler 103.

The water streams b, c and d shown and also the direct contact cooler 103 and the evaporative coolers 104 are integrated into a cooling water circuit denoted here by 10, which can also comprise any further water streams, pumps, direct and indirect heat exchangers, etc, which are not shown. For example, the main air compressor 102 can, as shown here in greatly simplified form, have at least two compressor stages 1 and 2 between which intermediate cooling by means of an intermediate cooler 3 occurs. Typical main air compressors 102 of air fractionation plants comprise from five to nine compressor stages and a corresponding number of intermediate coolers. Cooling water in the form of the stream s can be fed into the illustrated intermediate cooler 3 which is configured for indirect heat exchange. The stream s can, in particular, be a substream of the stream c, i.e. cooling water which likewise circulates in the cooling water circuit 10. An analogous situation applies to further (after-)coolers as explained below. Further water streams can be fed at any place into the cooling water circuit 10, for example in order to compensate for evaporation losses, as indicated here by the water stream e. Furthermore, cross-connections between water streams, regulating devices, measurement sensors and the like can be arranged at advantageous places in the cooling water circuit 10.

The central component of the cooling water circuit 10 is a recooling apparatus 11, which is shown here as a wet cooler and can be configured, for example, as a cooling tower having forced ventilation. However, as mentioned above, any other embodiments are also possible. The recooling apparatus 11 is configured for operation according an abovementioned embodiment of the invention. A stream f of atmospheric air having a wet bulb temperature prevailing at the location of the air fractionation plant 100 is fed to the recooling apparatus 11. The recooling apparatus 11 is, for example, configured for cooling water of a water stream g to be cooled, formed in the depicted example by the water streams d and e, to a temperature level which is not more than 3 K above the wet bulb temperature of the air stream f. This applies particularly when the wet bulb temperature of the air stream f is above 289 K.

The further processing of the compressed and cooled feed air stream a, which is now designated by h, corresponds largely to that in conventional air fractionation plants, for example in an air fractionation plant as described in H. W. Häring (editor), Industrial Gases Processing, Wiley-VCH, 2006, in particular section 2.2.5, “Cryogenic Rectification”:

The compressed and cooled feed air stream h is fed to an adsorber set 105 which comprises alternately operated adsorber vessels and can be regenerated by means of a regeneration gas stream i. The regeneration gas stream i can be heated by means of an electrically operated and/or steam-operated regeneration gas heating device 106. To provide the regeneration gas stream i, it is possible to use a stream k, the provision of which will be described in more detail below.

A compressed air stream which has been dried in the adsorber set 50 is denoted by l. Depending on the configuration of the air fractionation plant 100, the compressed air stream l can be provided at a pressure which makes after-compression necessary or dispensable (the latter in the case of High Air Pressure processes). In the example shown, a substream m of the compressed air stream l is fed to an after-compressor 107. An after-cooler, which is not designated separately, of the after-compressor 107 can likewise be cooled using water from the cooling water circuit 10.

The substream m and a substream n which has not been after-compressed of the compressed air stream l are, according to the embodiment depicted, fed to a main heat exchanger 108 and taken off from this at different temperature levels. The stream m can be depressurized by means of a generator turbine 109 and, after being combined with the stream n, be fed into a high-pressure column 111 of a distillation column system 110. Further substreams of the compressed air stream l can be formed, cooled, after-compressed, depressurized and likewise fed into columns of the distillation column system 110 in an advantageous way, for example a known throttle stream which is not shown here.

The high-pressure column 111 together with a low-pressure column 112 forms a double column system of a known type. In the example shown, the distillation column system additionally comprises an argon enrichment column 113 and a pure argon column 114, but these do not have to be provided. Further distillation columns can be provided.

The operation of the distillation column system 110 is known and will therefore not be explained. In the example shown, the distillation column system 110 is supplied, inter alia, with a gaseous nitrogen stream o, “impure nitrogen” in the form of the stream p, from which the stream k and/or a stream q can be formed after heating in the main heat exchanger 108 and can be fed to the regeneration gas heater 106 or the evaporative cooler 104, and a liquid, oxygen-rich stream r can be taken off. Instead of the stream q, it is also possible to use, for example, a cold, nitrogen-enriched stream. Further streams will not be explained in detail. Any streams can be heated in the main heat exchanger 108, compressed or pressurized upstream or downstream of the main heat exchanger 108, combined with other streams and divided into substreams.

FIG. 2 shows the average cooling water temperatures of the months of a year and the corresponding wet bulb temperatures in order to illustrate an embodiment of the invention. The cooling water temperature is plotted in K on the ordinate versus the wet bulb temperature in K on the abscissa. In the graph, the wet bulb temperature is shown in the form of the data points 201, the cooling water temperature in a conventional design of a recooling apparatus configured as cooling tower is shown in the form of the data points 202 and the cooling water temperature in a design according to an embodiment of the invention is shown in the form of the data points 203.

The conventional design results in a cooling limit difference of 8 K of a wet bulb temperature of 289 K. According to the embodiment of the invention depicted, the cooling limit difference is reduced by five kelvin to 3 K. The use of a more efficient cooling tower and thus a lowering of the cooling limit temperature leads to two effects, namely firstly colder cooling water and secondly a smaller relative difference between the cooling water temperature and the wet bulb temperature. This means that the efficiency loss of the cooling towers is fundamentally lower in relatively cold months for a design having a relatively small cooling limit difference. The reason for the lower efficiency loss of a large cooling tower in the cold months is the water/air ratio which can be shifted in favour of air. The mass flow of water is the same for both cooling tower variants, and the critical factor is that a larger amount of air can flow through the cooling tower in the case of a large cooling tower for the same amount of cooling water, and this air takes up the evaporating water and at the same time allows great convective cooling. This effect makes a positive contribution especially at low air temperatures at which the air can take up little water.

FIGS. 3A and 3B show the additional cooling of cooling water (FIG. 3A) made possible according to the invention and the associated energy savings (FIG. 3B). In FIG. 3A, a temperature difference in K and in FIG. 3B an energy difference in kW is plotted on the ordinate versus the months January (J) to December (D) on the abscissa.

As can be seen from FIG. 3A, virtually 5 K cooler cooling water is obtained on average. The energy saving which can correspondingly be seen in FIG. 3B is from 270 to 450 kW per month and leads to an annual average saving of 340 kW. The decrease in the compressor power consumption by 340 kW corresponds to 1.5% of the total compressor power consumption. 

What we claim is:
 1. An air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air, characterized in that the recooling apparatus is configured so as to cool the cooling water, at least at a wet bulb temperature of the cooling air of more than 289 K, to a temperature which is not more than 3 K above the wet bulb temperature.
 2. The air fractionation plant according to claim 1, wherein the recooling apparatus is configured so as to cool the cooling water to a temperature which is at least 0.5 K above the wet bulb temperature.
 3. The air fractionation plant according to claim 1, wherein the recooling apparatus comprises a cooling tower.
 4. The air fractionation plant according to claim 3, wherein the recooling apparatus has forced ventilation.
 5. The air fractionation plant according to claim 1, wherein the cooling water circuit comprises a heat exchanger which is arranged downstream of a compressor.
 6. The air fractionation plant according to claim 1, wherein a cooling zone range of from 5 to 25 K is provided.
 7. A method of operating an air fractionation plant in which a cooling water circuit having a recooling apparatus is provided for cooling compressed air, where the recooling apparatus is configured for cooling cooling water using cooling air, characterized in that the recooling apparatus is configured and operated so as to cool, at least at a wet bulb temperature of the cooling air of more than 289 K, the cooling water to a temperature which is not more than 3 K above the wet bulb temperature.
 8. The method according to claim 7, wherein the specific surface area of the packing and/or the ratio of liquid to gas and/or the pressure drop in the recooling apparatus is selected and/or set in such a way that, at least at a wet bulb temperature of the cooling air of more than 289 K, the cooling water is cooled to the temperature which is not more than 3 K above the wet bulb temperature. 